The lower and upper halves of the frequency spectrum of the COFDM modulation signal that convey the same coded data mirror each other in double-sideband COFDM (abbreviated as DSB-COFDM). The term asymmetric-sideband COFDM (abbreviated as ASB-COFDM) is used herein to specify COFDM in which the lower and upper halves of the frequency spectrum of the COFDM modulation signal that convey the same coded data do not mirror each other. DSB-COFDM and ASB-COFDM are respective species of COFDM dual-subcarrier-modulation (DCM).
Double-sideband COFDM or DSB-COFDM modulation of radio-frequency (RF) signals has been used several years for over-the-air broadcasting of DTV in accordance with the DVB-T and DVB-T2 Standards for Digital Video Broadcasting in several countries other than the United States of America and Canada. DSB-COFDM RF signals are being broadcast in the Republic of South Korea and in the United States of America in accordance with an ATSC 3.0 Standard developed by the Advanced Television Systems Committee, an industry-wide consortium of DTV broadcasters, manufacturers of DTV transmitter apparatus and manufacturers of DTV receiver apparatus. In over-the-air DTV broadcasting in accordance with any of these standards, the OFDM carriers are transmitted in DSB format as subcarriers of a principal RF carrier that is suppressed in amplitude to some degree.
Prior-art receivers for DSB-COFDM modulated RF signals, such as receivers for DTV broadcasting, have folded the frequency spectrum in half by synchrodyne to baseband before discrete Fourier transformation (DFT) and subsequent demapping of the quadrature amplitude-modulation (QAM) of COFDM signal subcarriers. The constructive combining of mirrored OFDM subcarriers improves the signal-to-noise ratio (SNR) of reception over an additive-white-Gaussian-noise (AWGN) channel by 6 dB.
Receivers that demodulate the lower and upper halves of the frequency spectrum of a DSB-COFDM modulated RF signal independently of each other, thus to avoid the folding of frequency spectrum in synchrodyning to baseband, are described in U.S. patent application Ser. No. 15/641,014 filed by Allen LeRoy Limberg on 3 Jul. 2017 and titled “Double-sideband COFDM signal receivers that demodulate unfolded frequency spectrum”. This patent application prescribed respective discrete Fourier transform (DFT) of the lower and the upper halves of the frequency spectrum of the COFDM modulation signal. The sets of QAM symbols from those two halves of that frequency spectrum are demapped separately, and then their corresponding QAM-map labels are diversity combined. (DSB-COFDM modulation affords some frequency diversity that can help receivers as described in U.S. patent application Ser. No. 15/641,014 to overcome some frequency-selective fading and narrowband interference, so long as neither affects the more central frequencies of the transmission channel.)
The diversity combining performed by the receivers described in U.S. patent application Ser. No. 15/641,014 includes combining soft bits of corresponding QAM-map labels in accordance with the respective amplitudes of their QAM symbols prior to channel equalization, thus to improve SNR of reception over an AWGN channel by 8.5 dB. This improvement is 2.5 dB better than the 6 dB improvement obtained by QAM symbol averaging, as obtains in synchrodyning procedures that fold the frequency spectrum of the DSB-COFDM signal in half. This 2.5 dB better SNR is in line with observations concerning multiple-in/multiple-out (MIMO) reception of COFDM modulation signals from plural-antenna arrays, as reported in U.S. Pat. No. 7,236,548 titled “Bit level diversity combining for COFDM system” issued 26 Jun. 2007 to Monisha Ghosh, Joseph P. Meehan and Xuemei Ouyang.
U.S. patent application Ser. No. 15/685,965 filed by Allen LeRoy Limberg on 24 Aug. 2017 and titled “Communication systems using independent-sideband COFDM” concerns transmitters and receivers for certain ASB-COFDM signals. In these certain ASB-COFDM signals the QAM of subcarriers in the lower-frequency half of the signal frequency spectrum convey the same coded data as the QAM of subcarriers in the upper-frequency half of the signal frequency spectrum. U.S. patent application Ser. No. 15/685,965 posits that symbol recombination techniques known to be advantageous in connection with transmitting coded data twice in respective DSB-COFDM signals can be adapted for securing similar advantages in connection with ASB-COFDM modulated RF signal that transmits similar coded data in both the lower-frequency and upper-frequency halves of its frequency spectrum. I.e., Limberg discerned that teachings in regard to techniques for hybrid automatic repeat request (HARQ) and for MIMO could have more general application, particularly with regard to ASB-COFDM modulated RF signal transmitting similar coded data in both the lower-frequency and upper-frequency halves of its frequency spectrum.
By way of specific example, U.S. patent application Ser. No. 15/685,965 pointed out that ASB-COFDM signals can be designed to transmit dissimilar respective mapping patterns in two sets of QAM symbols transmitted parallel in time. Such procedure is adapted from that known for mobile communications using QAM signals directly for transmission and reception. The dissimilarity in the respective mapping patterns in two sets of QAM symbols transmitted parallel in time is referred to as “labeling diversity”.
Labeling diversity (sometimes referred to as “symbol recombination”) can be designed to support a bit-reliability-averaging (BRA) procedure. Bits more likely to experience error in lattice-point labels of the mapping pattern for the first set of symbol constellations correspond to bits less likely to experience error in lattice-point labels for the second set of symbol constellations. Bits less likely to experience error in lattice-point labels of the mapping pattern for the first set of symbol constellations correspond to bits more likely to experience error in lattice-point labels for the second set of symbol constellations. The BRA procedure is implemented in a receiver by diversity combining pairs of corresponding labels from the first and second sets of symbol constellations, using a procedure known as “soft-bit maximal ratio combining” or “SBMRC”. Akram Bin Sediq and Halim Yanikomeroglu described such procedure in a paper titled “Performance Analysis of Soft-Bit Maximal Ratio Combining in Cooperative Relay Networks”, which was published in IEEE Transactions on Wireless Communications (Volume: 8, Issue: 10, October 2009), pp. 4934-4939. Receivers for two sets of QAM symbols transmitted parallel in time can thus be designed to exploit labeling diversity to achieve shaping gain significantly larger than that which can be secured from NuQAM—i.e., QAM with non-uniform spacing between labeled points in the QAM symbol constellation mapping. De-mapping QAM with uniform spacing between labeled points can be done more simply than de-mapping QAM with non-uniform spacing between labeled points. Also, NuQAM requires forward-error-correction (FEC) code rates not significantly greater than ½ in order to provide shaping gain, while BRA provides significant shaping gains at higher FEC code rates.
In June 2005 a paper “Symbol mapping diversity design for multiple packet transmissions” authored by Harvind Samra, Zhi Ding, Peter M. Hahn was published in IEEE Transactions on Communications Vol. 53, No. 5, pp. 810-817. The Samra et al. paper presented a simple, but effective method of enhancing and exploiting diversity from multiple packet transmissions in systems that employ nonbinary linear modulations such as phase-shift keying (PSK) and quadrature amplitude modulation (QAM). This diversity improvement results from redesigning the symbol mapping for each packet transmission. Symbol mapping diversity (SMD) requires a small increase in receiver complexity, but provides very substantial reductions of bit error rate when applied to additive white Gaussian noise (AWGN) and flat-fading channels. The general SMD concept was later incorporated in multiple-input/multiple-output (MIMO) and multiple-input/single-output (MISO) communication systems, but was referred to as “labeling diversity”.
Panasonic Corporation sent a paper titled “Enhanced HARQ Method with Signal Constellation Rearrangement” to the TSG-RAN Working Group 1 for discussion during its Meeting #19 held Feb. 27-Mar. 2, 2001 in Las Vegas, Nev., USA. Combining two 16QAM transmissions with labeling diversity between the labeling of the lattice points in their respective square 16QAM symbol constellations was reported to provide a 1.2 dB advantage over Chase combining two 16QAM transmissions without labeling diversity. Combining two 64QAM transmissions with labeling diversity between the labeling of the lattice points in their respective square 64QAM symbol constellations was reported to provide a 1.8 dB advantage over Chase combining two 64QAM transmissions without labeling diversity. Turbo coding rate was ¾, both for 16QAM transmissions and for 64QAM transmissions. The Panasonic Corporation paper was not directed to COFDM signals.
In Gray mapping of QAM and APSK, the plural-bit labels of immediately adjacent lattice points differ in only a single one of their bits. With regard to Gray mapping 16QAM symbols, it has been shown that a constellation rearrangement approach improves the performance if two or more versions of the same word are transmitted. The constellation rearrangement scheme for Gray mapping is based on different levels of reliability for the bits, depending on the position of the selected 16QAM symbols within the constellation. Consequently, the rearrangement rules focus on changing the location of the rearranged version of the QAM symbol to achieve an averaging effect of the levels of reliability. First and second sets of QAM symbols transmitted parallel in time are labeled such that the labeling of each set of QAM symbols bits more likely to experience error in the labeling of each set of QAM symbols is in accordance with the bits less likely to experience error in the labeling in the other set of QAM symbols. For details on constellation rearrangement for 16-QAM Gray mapping, one is referred to U.S. Pat. No. 7,920,645 titled “Data transmissions in a mobile communication system employing diversity and constellation rearrangement of a 16 QAM scheme” granted 5 Apr. 2011 to Alexander Golitschek Edler Von Elbwart, Christian Wengerter and Isamu Yoshi. U.S. Pat. No. 7,957,482 titled “Bit-operated rearrangement diversity for AICO mapping” granted 7 Jun. 2011 to Alexander Golitschek Edler Von Elbwart, Christian Wengerter and Isamu Yoshi describes more extensively the use of labeling diversity for more than one set of 16QAM symbols transmitted parallelly in time. (AICO is the acronym for “Antipodal Inverted Constellation”.) The Von Elbwart et alii patents were not directed to COFDM signals.
A 2015 paper “Labeling Diversity for 2×2 WLAN Coded-Cooperative Networks” authored by Saqib Ejaz, Feng-Fan Yang and Hong-Jun Xu was published in Radio Engineering, Vol. 24, No. 2, pp. 470-479. Wireless local area networks (WLAN) utilize OFDM signals, and this Ejaz et alii paper considers labeling diversity in QAM of OFDM signals employed in MIMO networks. This Ejaz et al. paper does not propose the application of labeling diversity to QAM of respective sets of OFDM subcarriers within a single COFDM DCM signal. This Ejaz et alii paper does aver that the general idea of labeling diversity can be extended to other high order modulation schemes besides 16QAM. This Ejaz et al. paper reports that labeling diversity has shown promising BER performance improvements in systems without labeling diversity, and that labeling diversity also lowers the Error Floor (EF) region by ensuring error-free feedback during the iterative decoding process.
Mappings that maximize labeling diversity in 16QAM symbol constellations were disclosed specifically by Maciej Krasicki in his paper “The essence of 16-QAM labeling diversity” published 11 Apr. 2013 in Electronics Letters, Vol. 49, issue 8, pp. 567-569. Maps for such a mapping will be referred to as optimal-SMD maps in this specification, its accompanying claims, and FIGS. 27 and 28 of its drawings. Krasicki described his work in more detail in a 20 Feb. 2015 open-access on-line Springerlink publication titled “Algorithm for Generating All Optimal 16-QAM BI-STCM-ID Labelings”. This publication focused on 16-QAM labelings of bit-interleaved space-time coded modulation with iterative decoding (BI-STCM-ID), describing an algorithm to generate (without need for random search) optimal labelings for BI-STCM-ID systems with any number of transmit and receive antennas, transmitting over a Rayleigh-fading channel.
U.S. patent application Ser. No. 15/685,965 pointed out that bit-reliability averaging (BRA) of QAM symbols can be advantageously applied to COFDM DCM signals, thus to advance such art significantly. As contrasted to transmitting one set of consecutive NuQAM symbols a single time to obtain shaping gain, BRA has one significant disadvantage. BRA involves transmission of two sets of consecutive QAM symbols conveying similar FEC-coded data, which double transmission halves data throughput for given channel bandwidth. However, BRA does not further reduce data throughput for given channel bandwidth in ASB-COFDM in which the lower and upper halves of the frequency spectrum of the COFDM modulation signal convey the same coded data. Data throughput for given channel bandwidth can be the same as for DSB-COFDM.
In some types of COFDM DCM signals, all pairs of subcarriers each conveying the same coded data have uniform spacing between the subcarriers of each pair. This enables receivers as described in U.S. patent application Ser. No. 15/685,965 to overcome some frequency-selective fading and narrowband interference, even if it affects the more central frequencies of the transmission channel. This reduces the need for forward-error correction coding of data to be done at as low a code rate as for DSB-COFDM.
Patent application US-20170104553-A1 published 13 Apr. 2017, titled “LDPC Tone Mapping Schemes for Dual-Sub-Carrier Modulation in WLAN” and claiming an original filing date of 11 Oct. 2016 for inventors Jian-Han Liu, Sheng-Quan Hu, Tian-Yu Wu and Thomas Edward Pare, Jr. describes a species of ASB split-spectrum COFDM modulation signal which utilizes dual subcarrier modulation (DCM). The DCM modulates the same information on a pair of subcarriers, which can be separated in frequency to improve frequency diversity in OFDM systems. Such separation is re improves reliability of reception, especially when there are narrow-band interferences, according to US-20170104553-A1. US-20170104553-A1 describes respective mappings of the sets of 16QAM symbols transmitted parallelly in time, which mappings are similar to each other.
Undesirably large peak-to-average-power ratio (PAPR) has long been a well-known problem in regard to over-the-air (OTA) multiple-carrier radio-frequency (RF) signal transmissions, such as the COFDM signals used for digital television (DTV) broadcasting. The average power of the DTV transmissions has to be held back substantially to avoid frequent occurrence of non-linearity and clipping in the amplifiers for COFDM symbols. Such effects cause undesirable spectrum spreading. Typically, average power is held back 10 to 15 dB or so. A variety of techniques to reduce PAPR in OFDM transmissions, so that average power need not be held back as much, have been proposed in the prior art. However, most of these techniques have at least one shortcoming and have not been used very much, if at all, in commercial OTA DTV broadcasting.
Simply clipping peaks of baseband COFDM signals is one technique used in the prior art to limit PAPR, but it introduces bit errors into the baseband COFDM signals recovered by a receiver. These bit errors are corrected insofar as possible during decoding of FEC coding. The need for such correction undesirably reduces the capability of the decoding of FEC coding to correct other errors in the received baseband COFDM signals, such as those attributable to accompanying noise or short-duration diminution in the strength of received signal. The clipping procedures tend to generate out-of-band radiation, which should be taken into consideration in the design of passband filtering for the COFDM transmitter. Also, there tends to be a problem with re-growth of peaks in the digital-to-analog conversion, which re-growth taxes subsequent band filtering procedures. If the coded data conveyed by the baseband COFDM signals has been randomized, very large peaks in their power are unlikely to occur as frequently, so clipping of them in the linear power amplifier of a transmitter may be tolerated if adequate band filtering procedures follow.
Selected portions of the transmitted COFDM signals can be transmitted at reduced power to reduce the energy of their peaks. Such schemes require both transmitter and receivers to be of more complex construction; and side information concerning the pattern of reduced power of transmission must be conveyed from the transmitter to the receivers. Such side information undesirably tends to reduce data throughput.
In other schemes the COFDM transmitter switches QAM symbols around in several patterns, the pattern that offers the lowest PAPR being selected for transmission. Such schemes also require both transmitter and receivers to be of more complex construction; and side information concerning the pattern of symbol switching must be conveyed from the transmitter to the receivers. Such side information undesirably tends to reduce data throughput.
To avoid the necessity of transmitting side information, other PAPR reduction techniques have been pursued, in which some of the OFDM carriers are used for PAPR reduction purposes rather than for data transmission. Reserved tones are inserted, the respective modulations of these dummy carriers being calculated so as to reduce PAPR. This comes at the cost of reduced data throughput, however. Typically this reduction in data throughput is of the order of 10% or so.
U.S. Pat. No. 8,040,963 titled “Method for reducing peak-to-average power ratio in an OFDM transmission system” claiming a 20 Oct. 2006 priority date was granted 18 Oct. 2011 to Ondrej Hlinka, Ondrej Hrdlicka and Pavol Svac. The patent describes PAPR reduction based on a complementary parity coding in which the coding rules are derived from an appropriate auto-correlation property of transmitted symbol sequences. The techniques were also described by P. Svac and O. Hrdlicka in a paper titled “A high peak-to-average power ratio reduction in OFDM systems by ideal N/2-shift aperiodic auto-correlation property” presented as part of the Joint IST Workshop on Mobile Future, 2006 within the Symposium on Trends in Communications '06 held 24-27 Jun. 2006 in Bratislava, Slovakia. This paper and U.S. Pat. No. 8,040,963 assert that a significant PAPR reduction of 6 dB, independent of the number of subcarriers, can be achieved in OFDM by assuring the appropriate auto-correlation property of transmitted data symbol sequences. Binary phase-shift keying (BPSK) data symbols were arranged in paired sequences, each successive pair of sequences being transmitted in a respective OFDM symbol. So, in an OFDM signal having a number N of subcarriers the data symbols conveying the same information are N/2 subcarriers apart. This procedure is a species of symmetric cancellation coding (SCC).
COFDM can use a technique symmetric cancellation coding (SCC) in which OFDM carriers are arranged in pairs, the QAM of each of the two OFDM carriers in a pair being antipodal to the QAM of the other. While such SCC is used principally for PAPR reduction, it is reported to reduce PAPR of COFDM in a paper titled “PAPR Performance of Dual Carrier Modulation using Improved Data Allocation Scheme” that Soobum Cho and Sang Kyo Park presented at the 13th International Conference on Advanced Communication Technology (ICACT2011) held 13-16 Feb. 2011 in Seoul, Republic of Korea. Their dual carrier modulation (DCM) spaces the OFDM subcarriers N/2 carriers apart to maximize frequency diversity, N being the total number of carriers in the OFDM signal. FIG. 4 of that paper shows PAPR of OFDM DCM being about 2.5 dB less than PAPR of conventional OFDM, when 16QAM of OFDM subcarriers is used. The labeling diversity of the two 16QAM mapping patterns Cho and Park used to secure lower PAPR does not support BRA, so as to optimize shaping gain.
Newer designs of COFDM transmitters for broadcast television improve power amplifier efficiency using variants of the methods described U.S. Pat. No. 6,625,430 titled “Method and apparatus for attaining higher amplifier efficiencies at lower power levels” granted 23 Sep. 2003 to Peter J. Doherty. Accordingly, the PAPR reduction techniques described supra have become less likely to be resorted to. However, the large PAPR of DSB-COFDM also causes problems in receiver apparatus which are not avoided (and indeed may be exacerbated) by using a Doherty method in the broadcast transmitter. These problems concern maintaining linearity in the radio-frequency (RF) amplifier, in the intermediate-frequency (IF) amplifier (if used) and in the analog-to-digital (A-to-D) converter.
Superposition coded modulation is described in detail by Li Peng, Jun Tong, Xiaojun Yuan and Qinghua Guo in their paper “Superposition Coded Modulation and Iterative Linear MMSE Detection”, IEEE Journal on Selected Areas in Communications, Vol. 27, No. 6, August 2009, pp. 995-1004. In superposition coded modulation (SCM) the four quadrants of square QAM symbol constellations are each Gray mapped independently from the others and from the pair of bits in the map label specifying that quadrant. Peng et alii studied iterative linear minimum-mean-square-error (LMMSE) detection being used in the reception of SCM and found that SCM offers an attractive solution for highly complicated transmission environments with severe interference. Peng et alii analyzed the impact of signaling schemes on the performance of iterative LMMSE detection to prove that among all possible signaling methods, SCM maximizes the output signal-to-noise/interference ratio (SNIR) in the LMMSE estimates during iterative detection. Their paper describes measurements that were made to demonstrate that SCM outperforms other signaling methods when iterative LMMSE detection is applied to multi-user/multi-antenna/multipath channels.
Jun Tong and Li Peng in a subsequent paper “Performance analysis of superposition coded modulation”, Physical Communication, Vol. 3, September 2010, pp. 147-155, separate superposition coded modulation into two general classes: single-code superposition coded modulation (SC-SCM) and multi-code superposition coded modulation (MC-SCM). In SC-SCM the bits in the superposed code layers are generated using a single encoder. SC-SCM can be viewed as conveying a special BICM scheme over successive SCM constellations. In MC-SCM the bits in the superposed code layers are generated using a plurality of encoders supplying respective codewords. MC-SCM can be viewed as conveying special-case multi-level coding (MLC) scheme over successive SCM constellations.
An important aspect of the invention described infra is recognition that there are previously unrecognized ways in which SC-SCM maps of QAM symbol constellations can be advantageously used in dual QAM mapping, wherein the same coded data is conveyed by two streams of QAM symbols that are differently mapped from each other. The use of dual QAM mapping for COFDM involves a form of dual carrier modulation (DCM) commonly referred to as dual-carrier COFDM or DC-COFDM, since each segment of coded data governs the modulation of two COFDM carriers. (Single carrier modulation is referred to as “SCM” in some other texts, but in this document the acronym “SCM” will be used exclusively to refer to superposition coded modulation.) SCM mapping of QAM symbol constellations in the lower sideband of ISB-COFDM and SCM mapping of QAM symbol constellations in the upper sideband of ISB-COFDM can be designed to reduce the PAPR of COFDM symbols by making the amplitude of the ISB-COFDM more uniform.